A direct or homodyne up-conversion scheme is one of the conversion techniques widely used in various types of communication systems due to its convenience and economic feasibility. FIG. 1 is a block diagram schematically illustrating a conventional direct up-conversion system.
According to a schematic operation of the conventional direct up-conversion system illustrated in FIG. 1, the conventional direct up-conversion system converts a digital baseband I signal and a digital baseband Q signal to an analog I signals and Q signals through a Digital to Analog Converter (DAC), respectively, removes a high frequency component from the analog I signals and Q signals by using a Low Pass Filter (LPF), and frequency modulates the I and Q baseband signals passing the LPF to Radio Frequency (RF) signals by using an I/Q modulator that is an analog device.
In the meantime, the I/Q modulator has an in-phase component I and a quadrature-phase component Q, in which I should accurately lie at right angles of 90° to Q. However, according to the direct up-conversion system, a phase imbalance that a phase between the in-phase component I and the quadrature-phase component Q is not 90° and a gain imbalance that a gain between the in-phase component I and the quadrature-phase component Q is differentiated are generated due to the characteristic of the I/Q modulator that is the analog device, so that the conventional up-conversion system has a disadvantage of the serious performance deterioration.
FIG. 2 is a graph illustrating an influence by phase/gain imbalance between the in-phase component I and the quadrature-phase component Q. As illustrated in FIG. 2, in a case where the phase/gain imbalance is generated between the in-phase component I and the quadrature-phase component Q, when a reception terminal demodulates a corresponding signal, a performance is seriously deteriorated by a mirror spectrum component. Accordingly, many techniques for measuring and correcting the I/Q phase/gain imbalance have been researched.
However, a technique for measuring and correcting an I/Q timing skew resulting from a delay time difference between I and Q components generated by a difference of DAC timings or PCB or cable lengths between I and Q components has not been researched much until recently. In this respect, in an OFDM communication system, such as actively used Wireless Local Area Network (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), or Long-Term Evolution (LTE) in recent days, using a broadband, a performance of the system is deteriorated due to the I/Q timing skew.
In consideration of the performance deterioration of the system, a conventional and general I/Q imbalance compensation method used in the direct up-conversion system employing the OFDM(A) employs a method of removing the I/Q imbalance in a final output terminal of the direct up-conversion system by measuring I/Q imbalance factor values of the direct up-conversion system and then inversely pre-distorting the measured I/Q imbalance factor values by a transmission unit. A method of measuring the I/Q timing skew, the I/Q phase imbalance, and the I/Q gain imbalance, which are the I/Q imbalance factors, is dealt with in another patent application of an invention different from the present invention, so that the description of the present invention is progressed based on an assumption that one has been aware of the I/Q timing skew, the I/Q phase imbalance, and the I/Q gain imbalance.
FIG. 3 is a block diagram schematically illustrating an I/Q imbalance compensation apparatus in the conventional direct up-conversion system. As illustrated in FIG. 3, the I/Q imbalance compensation apparatus in the conventional direct up-conversion system pre-distorts the I/Q phase imbalance and the I/Q gain imbalance in a digital terminal of a baseband signal by using the I/Q imbalance factors. However, even in this case, the pre-distortion is performed without the consideration of the I/Q timing skew, so that the efficiency of the pre-distortion is decreased.
FIG. 4 is a block diagram schematically illustrating an I/Q imbalance compensation apparatus in the conventional direct up-conversion system which performs the pre-distortion in consideration the I/Q timing skew in addition to the I/Q phase imbalance and the I/Q gain imbalance. However, it is also very difficult for the I/Q imbalance compensation apparatus of FIG. 4 to compensate for the I/Q timing skew in a baseband of a time domain.